staging: comedi: s626: use preferred kernel type u8

Fix the checkpatch.pl issue:
CHECK: Prefer kernel type 'u8' over 'uint8_t'

Signed-off-by: Saber Rezvani <irsaber@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>